Heat-treating element for heattreating furnaces



May 19., 1953 J. A. Dow 2,638,889

HEAT-TREATING ELEMENT FOR HEAT-TREATING FURNAcEs Filed May 8, 194? 2sheets-sheet 1 INVENToR. JOHN A. Dow

n Tram/ev May 19, 1953 J'. A. Dcw

HEAT-TREATING AELEMENT FOR HEAT-TREATING FURNACES Filed May 8, 1947 2Sheets-Sheet 2 mm-nrnrr-ez se.

L23 JOHN A. Dow

A T TORNEY Patented May 19, l1:953

A UNITED .l STAT-Es PATENT OFFICE.

* "f f n* 2,638,839 e' f Henares-Arise' ELEMENT roey HEAT- f 1 TREATINGFURNAcEs y John Antw, `Husein'gian weeds. Mich.; Doris B.

Dow, administratrix of saidJbhn -Ai Dow, de ceased, assignorto Doris B.Dow, fRoyal. Oak,

f Mich., as trustee [Appleman May, 8.31947, serai 10.146.124

This application is a continuation in Apart of the subject mattercontained in my copending applications for United States Letters Patent,Serial No.'664,1948 forLiquid Salt or Fusible 4lVleta-l Bath, led April25, 1945,v vnow abandoned, and Serial No. 670,436 for Heat'lreatingFurnace, sied May 17, 1946. Y Y y The invention relates toimprovements-in radi'- ant heating elements 'or tubesto be' employed inheat treating furnaces andfhas'particularrelation toheating elements rofthisl type in'whieh a combustible mixture is employed internally thereoffor the purpose of heating the *elements* o1' tubes to radianttemperatures.

In heating elementsy of this type as these have been constructedheretofore* lit has been a problem to construct the tubes so that theexterior surface thereof would be of unliffirm.y temperature from oneend thereof tothe other'. In such heating elements there has''arisen thedifficultyv that some parts lofthe `surface wereconsiderably hotter thanthe average temperature 'of the entire 3 Claims. (Cl. 126-91) crease inthe amount of floor space necessary for the installation of any numberof furnaces due to the fact that 4a considerable 'amount -of space "isvrequired at one side beside a furnace for operating and servicing theheating tubes. If'

this work has to be done from both sides -of a furnace,V then 'morespace must beutilized than Wouldotherwise be required.

It has been proposed 'heretofore Ato attempt to improve the performanceof such tubes by employingv devices therein to increase the velocityandthe turbulence and 4sinffafce*scrubbing action surface andconsequently these hot spots would burn out and hence makenecessarythereplacement of an element, when perhaps 'the' vlarger portion of theheating element was in condi-tion to render service for a considerablylonger-period of time. Also these 'hotspots-land Asuch 'irre-'guiar heatingmadeit'necessaryto'usea 'greater ameunt of heating f surface `in`any @givenfurnace v '-th'a'n would be required `Were 'it' possible toi'ernplw heating' elements "of" uriiforniA temperature rom one extremitythereof to another.Suoh'lael of.

uniformity of temperature in a 'furnaceemploy#l ing theseheatingvelements also caused ,nenunis formv heatingl of the work inradiation furnaces and-sometimes caused nonuniform heatingl o'f-theycirculzfiting mediuml in conve'ctici'n' furnaces. I

Not only have heating element-'slof this type been subject v'tononuniforr-A temperatures throughout the 'length thereof Itoapconsiderableextent but it 'has beencharacteristi-c thereof l'thatthe' tern-v perature of the surface-tended progressively' to decreasetoward-the 'outlet ends thereof. -Under such circumstanceslitjliasgenerally been considered not the best praeticeetoplacejallf'of- 'the tubes" in the 'furnace-trom -oneside Wall thereof.instead of suehL desirable practieef'it' has been necessary-hereto'f-oreto insert one rowl *of tubes Ain the furnace llrcm "oneside wall thereof "and another 'from thelop'pos'ite side ofthe lfurnace.This practice of 'inserting the tubes inthis Inan- -ner vfrom'oppositeside wallshastended t vide more lu'niiorn'ilnaating throughout "theentire -furnace, but it has required''a censiderable 'inof the heatingfluid Within the tubes in particular regions therein Where it wasconsidered desirable to -inerease'fthe surface temperature oftheelements. These expedients have been'objectionable from the standpointthat theycreated other -hot spots the heating elements which likewisereduced the 'life thereof. Also 'these expedients tended to greatlyreduce 'and render vnomir'iif'orrn the space provided -forthecombustible fluid With inthe heatingelements and'lconsequently made itnecessary either to make the heating elements largeror to tolerateV'in'eflicie'nt combustion of the combustible fluid employed.

such heat by radiation through the combustible uid to the heat radiatingsurface of the heating elements. Itis furthermore proposed` to. employrelatively uniform heat kabsorbing and radiating i means' Yofthischaracter which will not create excessive Yturbulence or scrubbingaction and. will not excessively increase anddecrease the velocity ofthe combustible nuid within `the heating .ele-

ment. y y.

Such means may be arranged Within the heating -elerne.rits in suchmanner as t0 progressively increase. the radi'atn of vheat to theexterior surface of the heating elements toward the outlet thereof sothat the heating .effect of the elements will remain relatively constantnotwithstanding the reduction' in 'temperature in such region of theIco'mbustible uid contained therein. y Alsosuch means 'may be employedmerely for the purpose of increasing the capacity of the heating tubesemployed in any furnace. ASuch increase in capacity will result from theimproved elliciency in absorbing and radiating heat resulting from thecombustion of fuelA supplied to the Vfuel inthe `tubes employing suchmeans.

One of the objects of the invention is to provide a heating elementhaving a temperature throughout the extent of the heat radiating surfacethereof in which the variation from the average temperature of thesurface of the heating element is reduced .-to a minimum. v f

Another object of the invention is yto increase the life of heatingelements employed in heat treating furnaces by eliminating hot spotsfrom the elements which tend to burn o ut and to cause replacement ofthe elements sooner than VWould otherwise be necessary. y

Another object of the invention isvto provide for more uniform heatingof radiant heating ele l ments from one end of such'felements to anotherso that all of the elements can be placed in a furnace from one side ofthe furnace, and as `4 within the heating elements employed there- 1n;

Figs. 8, 10 and 12 are longitudinal sectional views through heatingtubes or elements employing other forms of heat absorbing and radiatingmeans therein;` and v f Figs.\'9,f11 and 13 are cross sectional viewsthrough the structures disclosed by Figs. 8, l and l2 taken respectivelyin the planes of lines 9--9,

a consequence thereof the floor space required for heat treating Workwill be reduced.

the capacity of radiant heating tubes employed in such furnace byincreasing the heat transfer rate from the products of combustion to theheat radiating surfaces of the tubes, and consequently the rate at whichheat may be transferred from said surfaces to the work being processed.

A still further object of the invention is to increase the combustionefficiency within radiant heating tubes of such type by increasing theamount of heat absorbing and radiating surfaces Within the tubes andwith which surface the burning gases are brought into intimate contactduring the combustion process.

Other and further objects of the invention will be apparent by referenceto the accompanying drawings of which there are two sheets, which bywayof illustrationshow preferred embodiments and the principles thereofand what I now consider to be the best mode in which' I havecontemplated applying these principles. Other embodiments of theinvention embodying the same or equivalent principles may be used andstructural changes may be made as desired by those skilled in the artWithout departing from the present invention and thepurview of theappended claims. I also contemplate that of the several differentfeatures of my invention, certain ones thereof may be advantageouslyemployed in some applications separate and apart from the remainder ofthe features.

In the drawings:

Fig. 1 is a cross sectional view of a heat treating furnace employingheating elements -or tubes embracing the principles of the invention;

Fig. 2 is a longitudinal sectional view through a heating element suchas those employed in the structure disclosed by Fig. 1; l Fig. 3 is alongitudinal sectional view of a modied form of heating element such asmight be employed in the structure disclosed by Fig, l; Figs. 4 and 'Iare longitudinal sectional views through the outlet sections of heatingtubes such as those disclosed by Figs. 2 and 3 and illustrating otherforms of heat radiating and absorbing means that may be employedtherein;

Fig. 5 is a cross sectional View of the heating tube disclosed by Fig. 4taken substantially in the plane of line 5 5 thereon;

Fig. 6 is a cross sectional view of a lower portion of the furnaceillustrated by Fig. 1 disclosing the transversely disposed passagesformed in the lower Wall in which the heating elements are disposed anddisclosing another form of heat radiating and absorbing means which maybe employed II-II and I3-I3 thereof. n Referring particularly to Fig. l,the numeral Hl indicates a heat treating furnace of conventional designcomprising side walls Il, a lower wall I2,

and an uppervwall I3. An opening in the front wall, not shown, providesmeans for inserting work baskets or containers I 4 within the heattreating compartment I 6 provided by the walls of the container, thesework baskets being supported on ai supporting rack I'I having wheels orrollers I8 mounted thereon, on which the Work baskets are adapted toroll easily into and out of the furnace. l

The heating chamber I6 of the furnace I8 is adapted to be heated by aplurality of heating elements indicated at 20, the heating elementsbeing dividedfinto an upper row indicated at I9 and a lower rowindicated at 2I. These heating elements may be positioned in the furnacein any desired angular position, although in the structure shown by Fig.1 they are positioned horizontally in spaced relation to one anotherfrom front to rear of the furnace and are provided with removable headsindicated at 22 adapted to be inserted in openings 23 formed in one ofthe furnacenside walls I I It` Will be noted that the openings 23 areall formed in the same side wall I I of the furnace ID therebypermitting the changing of all of the tubes or the servicing oradjustment thereof from the same side of the furnace. Since this workrequires considerable space, vthere will be a considerable saving in theover all space required for a number of furnaces due to the fact thattwo furnaces can be placed very close together when all of the heatingelements are available from the adjacent sides thereof.

In the form of heating element disclosed by Fig. 2 the heating elements20 comprise U-shaped tubes 24 similar to the tubes disclosed bycopending application Serial No. 670,436 previously referred to. In suchstructure the tubes 24 each comprise a straight tubular inlet section26, a straight tubular outlet section 21, the two sections beingconnected at one end by a curved tubular section 28. The inlet section26 has an inlet end 29 projecting through the head 22 and into which isintroduced by a conduit 3| the combustible gas employed in heatingfthetubes. Inside the inlet 29 and disposed in spaced and concentricrelation thereto is a short section of tubing indicated at 32, theinterior of which is connected to a conduit 33 employed in introducingair required to form the combustible charge within the interior of thetube 24. The air emerges from the inner end of the tube 32` and theremixes with the combustible gas supplied by the conduit 3| to theinterior of the inlet 29 of the tube 24. Within the interior of the tube32 there is concentrcally disposed a smaller and longer heat absorbingand radiating means, member or tube 34 which is adapted to extend fromone end to another of the straight section 26, the opposite end thereofbeing secured in an opening formed at 36 in the curved section 28. Thetube 32 projects putwardly beyond the end of the straight detestetubular -se'etioh zit-,good 'thespdeebetweeh the' tubes et theendofrtheseeti'oh {z ffls closed byftih annular cop 31. The -tubesaz andterminate in' the seme region lun-d the I'erids thereof are liliew'sje'closedb'y'd'dlso738.L ,l l -f y 1 f While such is essentiel, lthe spacewith-ih the tube 34 may be employed"lrlf-fdrodueingia controlled gas'ettiV pher'e to be'oirohldtediihthe heating eliamber tu; 'fassuoh spadeis employedih the structure disdlosed by my opehdihg appli-otttin"Serltl No.6'70;436. Hgwver; 'the 'til-bef :ea't' be util-ized shtml?lf'for 'radietingfahd dbs'obl heetehd for transferring this heezt by rfalotion to the 'section 26 orthetube 'n' for the fi'oiirposeshereinstated. 'l

Beyond the 'inlet 29' the sldethe tube 24 -eonipri'ses a ooihbtlstlo eamber ihditedjajt '39, the outlet to 'whifeh isiindieete'dfbythe'humoralu. This outlet fori-ris lilffp'ortion ofthe straight section `l1 whichlprojeots youi-wetreilly of the iur: heee through titel-lead i212.Thee'oihbustib'le funx; ture to be burnedihtheioorrlbstio heim er afs issupplied to the 'oohduitsfal lon-d 3:5. endjlgriited in thecombl'lstlollhdnbrbyy suitable me'grls for 'this purpose. not shown. Themixture the tubes is generally igh-ited lmerely "by 'throwlhgje lightedmetoh lrlt'o theoutl'et'eridof e tubeiwheri the misrtu're'is turned on.Also 'the products of eombustion'diseharged from 'the' outlet t] irioybecre'd away a-Ild ictlltlgd `'tol the v`"elf/ifiio'sfpll'e by esuitableexheust'ihdnifoldjhot showt.; y 'AS is indicated.bytheidrawlnegjthe ooAbustin of the eofhbustlblejhlixture ihtrod'eed into he tbe 2iObousplihpa11yil1`tht jghtjsec n motion or thefoo'lbustible huid withinthe'ntbe 28.' Also when the products'of combustion pass beyond the endsofthe tubes 43, 44, 46 and-",V the temperaturelthereof;can be maintainedat the-lowest possible value-thatthe temperature of such uid can be andstill maintain the temperature of -the section21 at a substantiallyuni-` form value. Y

It will be noted that the combustible fluid, practically from one lendto another of the tube 24, encounters only relatively smooth surfacesand passages of somewhat different but relatively uniform cross section.This tends to cause the combustible fluidto flow in relatively straightlines, thereby producing a relatively uniform rate of combustion in thefluid-during-the period of combustion of the fluid within vthecombustion chamber 39. Thereafter thev heat absorbing and radiatingsurface is increased within the cornbustion chamber so as to absorb andradiate heat at an increasing rate during the period just prior to thedischargerof the products of combustion fromthe tube 24. During suchperiod the combustible fluid is not luminous to as great. an extent asbefore and consequently more surface is required to absorb the heat fromthe products of combustion and more surface is required to radiate suchheat to the section 21 to maintain a'relatively constant rate ofradiation from such section from one end to the other thereof.

In the structure disclosed by Fig. 3, where corresponding referencenumerals are employed to indicate elements similar to `those disclosedby Fig. 2, the straight section 26 is employed as a combustion chamberthroughout the entire extent of the cross sectional area thereof, andasthe luminous products of combustion pass therethrough such productsradiate heat to the interior surface of the section at -asubstantiallyuniform rate. The heat radiating means or member 42d employed in thesection 21 comprises open end tubes 48, 49 and 5| similar to the heatradiating and absorbing tubes employed in Fig. 2 except that there is aless number ofA said tubes and the centrally disposed tube is somewhatlarger and shorter. Such variation in the heating elements may be madewhen it is not desired to employ means such astube 34 in the inlet endof the combustion chamber 39 and when Va Alesser amount of heatabsorbing and radiating surface is desirable in the outlet end of suchcombustion chamber. In this structure any suitable means such as -thatemployed in the structure disclosed by Fig. 2 may be employed forsupplying combustible fluid to the inlet end 29 of the heating element26.

Figs. 4 and 5 disclose a section of a heating element or tube 24 havinga heat absorbing and radiating member or means 42 disposed thereinconsisting of a pair of strips of sheet metal 54 and 56. The strips 54and 56 each are bent along a middle longitudinal element thereof toprovide portions 51 and 58 disposed at right angles to one another. Thestrips may be welded together at 59 along the longitudinal bent portionsthereof so as to secure the strips together to provide a single unit,the strips being secured together in such manner that the portions 51and 58 of the two strips extend radially outwardly from the axis of thesection of the tube 24 at right angles to one another. The portions 51and 58 are cut away as is indicated at 6l to provide longitudinallyextending portions of decreasing width, the presence of which in theelement 42"` provides a progressive increase in the amount of metal andsurfacecomprising the element 42' from the inlet toward the outletendsthereof. e Ifheportions 6l vterminate a short distance from theoutlet end of theelement 42 to provide supporting flat portions 63 atone end thereof so as to support the member 42 Within the interior ofthe section of the vtube v24. Itwill be apparent that as the hot gasesor products of combustion'within the tube 24 pass along the smoothexposed surfaces of the element 42', the element surfaces will absorbfrom the gases a progressively increasing amount of heat and willthereafter radiate such heat to the interior surface of the tube 24 fromwhence it will be picked up'by convection or radiated to the circulatingmedium or work being. processed within a furnace in which the tubeisemployed.

In the structure disclosed by Fig. 6 there is employed in the tubes 24 aheat absorbing and radiating member 42 consisting of a plurality oftubes 64, 66 and 61, disposed one within another and being respectivelyof decreasing diameters toy provide parallel crescent shaped spacesbetween the tubes to providefor the flow of heated gases or products ofcombustion within the tubes. It .will be noted that the tubes 64, 66 and61 are not concentrically disposed within the heating tube or element 24but extend along the bottom of the tube 24 in such manner that thesurface of one tube rests upon and is tangent to the surface of anotheralong a longitudinal portion of the length ofthe tube 24. When locatedin such position the tubes 64, 66 and 61 can merely be placed within oneor more sections of atube 24 without requiring anyv supporting means orspacers for properly positioning the tubes within the vheating element24.

In Fig. 6 the heatingtubes 24 are positioned in transversely disposedwells 65 formed in the heat insulating fire brick or other suitablematerial of which thebottom or lower wall l2 with whichk a furnace -suchas that disclosed by Fig. l is provided. Due to the nonconcentricposition of the tubes 64, 66l and 61 in the section of the tube 24, asdisclosed by Fig. y6, it will be apparent that the closer proximity ofthesurfaces of the tubes 64, 66 and 61 adjacent the bottom portion ofthe tube 24 will tend to decrease the amount of heat-which can beabsorbed and radiated by such surfaces Vin such region.A In other Words,the top longitudinal portions of the tubes 64, 66 and 61 will absorb andradiate a maximum amount of heat due to the greater separation of thesurfaces of the tubesand the greater amount offgases which may affectsuch surfaces.

From the tops of the tubes the amount of heat which it is possi-ble forthe tubes to absorb and radiateprogressively decreases toward the bottomof thetubes due to the progressively closer spacing of the surfacesthereof and the progressively decreasing amount of the products ofcombustion which may affect such surfaces. The tubes.64, 66 and 61therefore providea directional heating effect in the heating element 24in which they are employed to such an extent that a minimum amount ofheat will be radiated bythe heating element 24 to the portion of thebottom wall l2 directly beneath the heating element, a progressivelyincreasing amount of heat laterally to the side walls of the Well 65 anda maximum amount upwardly in the direction of the work containers I4.

Fig. '7 discloses a section of a heating element or tube 24 providedwith a heat absorbing and radiating element 42a consisting of aplurality of concentric tubes 68,' 69 and 1| disposed therein in suchmanner that all the tubes terminate di- "surface temperature alongtheoutlet portion of said heating tiibe relatively uniform and at avalue which approximates the average tempera.- ture of said'tube alongthe inlet portion thereof, said radiant heat absorbing and radiatingmeans comprising an elongated thin-walled structure dividing theinterior of said heating tube into a plurality o-f substantiallyparallel paths of flow for the products of combustion withoutmaterial-1y obstructing or deflecting the flow of said products of combustion andaffording an extended radiating Asurface -presentf'ed toward but spacedfrom the heating 'tube wall.v

2. A heating element for-heat treating furnaces comprising a heatingtube adapted to be disposed within a furnace for heating Work to bedisposed therein, the interior of said heating tube providing acombustion chamber for the combustion of a gaseous combustible mixtureto be supplied thereto, the exterior of said heating tube providing aheating surface for yheating said work in response to the combustion ofsaid mixture therein, said heating tube being provided with inlet andoutlet means for supplying thereto 'and discharging therefrom saidmixture and the products of said combustion respectively, and radiantheat absorbing and radiating means disposed within the outlet portion ofsaid combustion chamber for increasing the heating effect of saidheating element at said portion, said radiant heat absorbing andradiating means comprising" an elongated thin-walled structure dividingthe interior of said heating tube into al plurality of substantiallyparallel paths of ilow for the products of combustion without materiallyobstructing or defiecting the flow of said products of combustion andaffording an extended radiating surface presented toward but spaced fromthe heating tube wall, said means being formed of a plurality of sheetmaterial members having opposite surfaces parallel to oneV another andwith longitudinal portions thereofl parallel to the interior surface ofsaidheating tube, said members being of different lengths and arrangedto provide increasing heat absorbing and radiating areas from the inletend to thev outlet ends thereof so as to tend to maintain the' exteriorsurface ltemperature along the tube at the-outlet portieri thereof relaa=tively uniform and' at a value-which approximates the averagetemperature -of the tube at the inlet portion-thereof.

3. A heating element for heat treating furnaces comprising a heating-tubeadapted to be disposed within a furnace for vheating Work to bedisposed therein, the interiorof said heating tube providing acombustion chamber for the combustion of a gaseous combustible mixtureto be supplied thereto, the exterior of said heating tube providing aheating surface for heating said Work in response to the combustion ofsaid mixture therein, said heating tubebeingi'provided with inlet andoutlet means for'supplying thereto and discharging therefrom saidmixture and the products of said combustion respectively, and radiantheat absorbing' and f radiating Ameans disposed within the outletportion of said combustion chamber for'increasing the heating effect ofsaid heating element at the outlet portion thereof so as to tend tomaintain the exterior surface temperature along the tube at the outletportion thereof relatively uniform and at a value which approximates theaverage temperature of the tube at the inlet portion thereof, said meanscomprising a plurality of sheets of material having parallelopposite-surfaces, said sheets being of diiferent lengths, said lengthsdecreasing from. said inlet towardsa'id outlet.

" JOHN A. DOW.

References Cited in the file'of this patent y UNITEDSTATES PATENTSNumber'

